1. Field of the Invention
The present invention relates generally to an OFDM (Orthogonal Frequency Division Multiplexing) communication system, and in particular, to a receiving apparatus and method for efficiently recovering cyclicity between symbols in an OFDM communication system.
2. Description of the Related Art
To support data rates required for future-generation mobile communication services, OFDM has recently been considered as a fundamental technology for the future-generation mobile communication network.
FIG. 1 is a block diagram of a transmitter in a conventional OFDM system. Referring to FIG. 1, a channel coder 101 encodes input data d(k) and an interleaver 102 interleaves the coded data. A signal mapper 103 converts the interleaved signal c(i) to signal vectors X(n, 0:N−1). An IFFT (Inverse Fast Fourier Transformer) 104 outputs transmission signal vectors x(n, 0:N−1) for the input of X(n, 0:N−1). A CP (Cyclic Prefix) inserter 105 inserts a guard interval into x(n, 0:N−1). The resulting signal is transmitted through a parallel to serial (P/S) converter 106 and finally an antenna.
The OFDM system inserts a CP between every adjacent symbol pair in the time domain in order to handle multipath fading. Further, in order to completely eliminate inter-symbol interference (ISI) and inter-channel interference (ICI) caused by the multipath fading, the length of the CP must be longer than a channel impulse response (CIR).
FIG. 2 illustrates a structure of an nth symbol when a total number of sub-channels is 8 (N=8) and the CP length is 4. FIGS. 3 and 4 illustrate signal receptions when the CP is as long as the CIR and shorter than the CIR, respectively.
If a channel with a CIR length of 4 is defined as h(D)=h0+h1D+h2D2+h3D3+h4D4, then an nth signal is received as illustrated in FIG. 3.
In FIG. 3, the CP length is equal to the CIR length. r(r, 0:7) except for a CP, r(n, −4:−1) in the received symbol is a circular convolution of x(n, 0:7) and h(D) That is, r(n, −4:−1) is CP, such that it is removed from the received symbol and the remained part r(r, 0:7) becomes a circular convolution of x(n, 0:7) and h(D). Therefore, orthogonality is maintained between sub-channels, thereby avoiding ISI and ICI.
FIG. 4 illustrates the structure of a received symbol when the CIR length is 4 and the CP length is 2. Referring to FIG. 4, as many samples r(n, 0) and r(n, 1) as the difference between the CIR length and the CP length contain (n−1)th symbol components, thereby causing ISI, which is illustrated in the shaded squares of FIG. 4.
Because using a CP decreases the frequency efficiency of the OFDM system, many studies have been conducted on methods of efficiently eliminating ISI and ICI, while minimizing the use of the CP. As a result, iterative cancellation methods have been proposed such as residual ISI cancellation (RISIC) for canceling insufficient CP-caused interference.
According to the RISIC, recovery of the defective samples involves elimination of the ISI component and recovery of the CP. In this case, recovered samples r′(n, 0) and r′(n, 1) can be expressed as shown below in Equations (1) and (2).r′(n,0)=r(n,0)−r3(n−1,7)−r4(n−1,6)+r3(n,5)+r4(n,4)  (1)r′(n,1)=r(n,1)−r4(n−1,7)+r4(n,5)  (2)
The subtraction of r3(n−1, 7) and r4(n−1, 6) from the received signal r(n, 0) in Equation (1) and the subtraction of r4(n−1, 7) from the received signal r(n, 1) in Equation (2) are equivalent to ISI cancellation. The addition of r3(n, 5) and r4(n, 4) to r(n, 0) and the addition of r4(n, 5) to r(n, 1) are equivalent to CP recovery. The CP recovery is repeated along with detection of x(n, 0:7).
However, the conventional ISI cancellation method, such as the RISIC, effectively recovers a CP only if a CIR is shorter than an OFDM symbol period, that is, when interference power is much less than signal power, an effective CP recovery is possible.
Another shortcoming of the conventional ISI cancellation method is that because a current symbol is estimated and a CP is recovered using the symbol estimate, when a long channel delay leads to a high interference power, reduction of interference power by CP recovery cannot be expected due to errors in the symbol estimation.
While various methods have been proposed using techniques of SISO (Soft-Input Soft-Output) channel decoding, optimal detection filtering, and leaked signal energy spread to the next symbol to overcome the above shortcomings, a SISO channel decoder demonstrates a very slight performance improvement under an SER (Symbol Error Rate) and the optimal detection filtering requires a complex process of inversion of a channel transmission function matrix in an initial stage. Additionally, an ISI combiner using the leaked signal energy spread needs estimation of the next transmitted symbol.